We have been studying selected immunohistological staining patterns of intraventricular tissue grafts in host brains and adrenal medullae in situ and in vitro. We have attempted to determine the essential factors involved in the development and survival of these tissues, in host brains after transplantation. Results indicate that the expression of cell adhesion molecules (CAMs) in transplanted cerebella is relatively undisturbed. Therefore, it is likely that in grafts, the molecular mechanisms of granule cell migration operate independently of normal inputs. Other results indicate that genetically normal cerebellar grafts can survive in a mutant host environment defective in CAMs and myelination. We have found that intraventricular grafts, as compared to intraparenchymal grafts, are not as well integrated with host brains in terms of the presence of interconnecting neurites. Other results indicate that CAMs are present in rat adrenal medullae in situ. In tissue cultures the outgrowth of fibers on chromaffin cells is associated with an enhancement of L1/Ng- CAM and N-CAM on their neurites. Transplanted, surviving adrenal medulla fragments within the lateral ventricle of the host rat demonstrated an enhancement of L1/Ng-CAM expression which was accompanied by a reorganization of the closely associated extracellular matrix. Other data further suggest that only in xenografts does an intense host reaction occur that is capable of destroying transplanted tissue. MHC class II immunoreactivity is enhanced in host parenchyma on microglial cells, but not on GFAP positive astrocutes after iso, allo and xenografting. Moreover, chronic CSF infusion of antibodies against MHC II molecules did not enhance the survival of intracerebral xenografts but instead produced granuloma reactions.